Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data that are readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided into a series of data tracks that extend circumferentially around the disk. Each data track can store data in the form of magnetic transitions on the disk surface.
The disk drive further comprises an interactive element, such as a magnetic transducer, that is used to sense the magnetic transitions to read data, or to conduct an electric current that causes a magnetic transition to write data. The magnetic transducer includes a read/write gap that positions the active elements of the transducer at a position suitable for interaction with the magnetic surface of the disk.
The disk drive also includes a slider that mounts the transducer to a rotary actuator arm, typically via a flexure element arranged between the slider and actuator arm to accommodate movement of the head during operation of the drive. The actuator arm operates to selectively position the head, including the transducer and slider, over a preselected data track of the disk to either read data from or write data to the preselected data track of the disk, as the disk rotates below the transducer.
In modern disk drives, the slider is configured to include an air bearing surface that causes the head, and thus the transducer, to fly above the data tracks of the disk surface due to interaction between the air bearing surface of the slider and fluid currents that result from the rotation of the disk. The amount of distance that the transducer flies above the disk surface is referred to as the "fly height." As should be understood, due to operation of the air bearing surface, the transducer does not physically contact the disk surface during normal read and write operation of the disk drive to minimize wear during operation of the drive.
The fly height for a slider refers to the height reached by the slider when the disk is rotating at its operational rotational velocity, i.e. the number of rotations per minute (rpm's) at which the disk drive was designed to operate. The fly height is designed to be at a level sufficient to insure that the transducer is spaced from the disk surface a distance suitable to maintain negligible contact between the head and disk surface during normal disk operation. In any disk drive product, the surfaces of the disks are typically not perfectly smooth and flat. There are peaks and valleys formed on the disk surface. The design fly height should be sufficient, e.g. to generally avoid head/disk contact, despite the passage of disk surface peaks below the head.
When the disk drive is not operating, the rotation of the storage disk is stopped, and the air bearing surface of the head does not act to cause the transducer to fly. Under such circumstances, the head, including the transducer, comes to rest on the disk surface. Typically, the actuator is operated prior to power down of the disk drive, to position the head over a landing zone provided on the disk surface at a location spaced away from any of the data tracks.
In a known contact stop operation of a disk drive, at power down of the drive, the fly height of the head gradually decreases as the rotational velocity slows, until the head comes into contact with the disk surface at the landing zone. The rotational velocity of the disk at which a head first contacts a disk surface is referred to as the "landing" velocity. Thereafter, the head remains in contact with the disk surface until and after rotation of the disk comes to a complete stop. The use of a landing zone prevents any damage to data tracks that may occur due to contact between the head and the disk surface. However, any contact between the head and the disk surface may result in damage to the transducer, and, in any event, contributes to wear of the head and disk surface.
This is also true when the disk drive is started again in a contact start operation. A contact start operation causes the commencement of rotation of the disk while the head is still in contact with the landing zone. The head remains in contact with the disk surface during acceleration of the disk, until the rotational velocity of the disk reaches a "take-off" velocity. The take-off velocity is the rotational velocity of the disk at which the air bearing surface first acts to lift the head from the disk surface such that contact between the slider and the disk surface is negligible. The take-off velocity is approximately equal to the landing velocity.
It is a goal of disk drive manufacturers to limit wear caused by contact between the head and disk surface, particularly during contact stop and start operations, to assure a more reliable mechanical performance of the disk drive. To that end, disk drive designs seek to accomplish disk drive operation wherein a head commences flying operation within an acceptable margin of rotational velocity measured from the operational rotational velocity for the disk.
For example, it has been determined that mechanical performance for a particular drive is likely to be acceptable for a relatively long work life when the head lifts off from and lands on the disk surface at a rotational velocity equal to approximately seventy per cent of the operational rotational velocity of the drive. In general, the fly height of a head is proportional to the rotational velocity of the disk, once the take-off velocity has been reached. In other words, the faster the disk is spinning, the higher the fly height of the head. It has been found that if flying operation (the take-off velocity) for a head is achieved by, e.g., seventy per cent of the operational velocity, the desired fly height should be reached by the head when the disk is accelerated up to the operational velocity. If the take-off velocity is greater than seventy per cent of the operational velocity, there is a significant likelihood that the head does not reach the desired fly height when the disk is accelerated to the operational velocity, leading to excessive wear and premature mechanical failure of the disk drive.
In addition, when the take-off or landing velocity is greater than seventy per cent of the operational velocity, the head remains in contact with the disk surface (i.e. the total sliding distance of the head on the disk surface during either a contact start or stop operation) for a total length of disk surface that is likely to result in excessive wear of the head/disk interface.
During manufacture of disk drives, it is desirable to be able to test each individual disk drive to determine the take-off and/or landing velocity for each head in the drive as a quality control procedure to insure that each head in the disk drive is operating to lift off from or land on the disk surface with a minimal sliding distance, and, in a contact start operation, is operating to reach the design fly height at the operational rotational velocity of the disk. However, there is not presently available a reliable and efficient system or method to determine take-off velocity values on a drive-by-drive basis, particularly for disk drives having multiple disks and heads.
Previous proposals involve recording a signal having a known frequency on a preselected track of the disk while the disk is rotating at a preselected rotational velocity and then reading back the signal at one or more different rotational velocities of the disk, including a rotational velocity corresponding to an expected take-off velocity. Contact between the slider and disk surface affects the signal read back from the disk. Knowledge of how the read back signal is affected can be applied to analyze the signal for evidence of slider/disk contact. The analysis can include frequency or amplitude demodulation of the read back signal for indications of slider/disk contact, and, e.g., measurement of the rotational velocity at which the read back signal first indicates slider/disk contact. Such a velocity measurement will generally correspond to the take-off velocity of the particular slider.
The methods and apparatuses described in the previous proposals suffer from shortcomings such as limited bandwidth for effective demodulation of the read back signal, time consuming procedures for implementation, particularly in disk drives having multiple heads and poor signal-to-noise ratios diminishing the accuracy and sensitivity of the detection process.